1. Field of the Invention
This invention relates in general to improved semiconductor devices or components and, more particularly, to improved transistors, thyristors and gate controlled switches.
2. Background Art
In power devices which handle heavy currents, the switching performance is very dependent upon differences in the lateral resistance. In transistors this would correspond to the lateral base and emitter resistance and in thyristors and gate controlled switches to the lateral gate and cathode resistance. For the purposes of this invention, the words "emitter" and "cathode" are used equivalently to stand for a principal injecting region of the device, and the words "base", "gate", and "control region" are used equivalently to stand for a principal controlling region of the device.
Power devices are sensitive to local variations in lateral resistance because such variations produce local variation in the lateral voltage drop. The varying localized voltage drop provides a different bias to different device regions which in turn causes conduction non-uniformity. During switching this conduction non-uniformity can lead to current crowding. There is a tendency for localized points of high conduction to enter regenerative thermal runaway or, at least, to require much higher switching drive. The problem is particularly severe in fast switching thyristors and gate controlled switches, i.e. gate turn-off thyristors, since charge injection or extraction during turn-on and turn-off is very sensitive to differences in the lateral distance, and hence resistance, between the gate electrode and cathode.
The involute spiral cathode-gate geometry has been applied to circular thyristors and gate controlled swithes to improve their switching performance. See for example, "An Involute Gate-Emitter Configuration for Thyristors," H. F. Storm et al., IEEE Transaction on Electron Devices, Vol. ED-21, No. 8, August 1974, pages 520-522, and "Semiconductor Power Devices," S. K. Ghandi, John Wiley and Sons, New York, 1977, pages 217-219 and 231-236.
An involute spiral resembles a spoked wheel in which the rim has been rotated through a predetermined angle while the hub is held fixed. However, unlike an ordinary radial spoke configuration, the involute spiral has the property that adjacent spokes are everywhere equidistant, independent of radius. Thus when the cathode and gate are arranged as involute spirals, the lateral cathode-gate spacing is everywhere constant. This arrangement gives improved device performance.
However, despite these improvements, prior art involute type devices have been expensive to manufacture because this configuration has been applied only to circular die. Circular die require additional shaping steps and waste substantial amounts of starting material since they must be cut from larger wafers. Both factors increase manufacturing costs. While prior art involute designs have been well adapted for circular die, they have not been suitable for use on rectangular or square die which are easier to shape and less wasteful of wafer area. Square die thyristors or gate controlled switches have continued to use conventional square "washer" shaped cathodes or "interdigitated finger" cathode-gate arrangements which do not provide the improved switching performance possible with an involute spiral cathode-gate structure. Furthermore, prior art devices continue to be limited in performance because only portions of the cathode-gate electrodes are thermally coupled to the conductive heat spreader used to extract heat from the front face of the semiconductor die or device. An air gap separates the remaining portions from the heat spreader of the typical pressure mounted package.
Accordingly, it is an object of the present invention to provide a semiconductor component in square form having improved switching performance while still making optimal use of the available device area.
It is an additional object of the present invention to provide an improved semiconductor component having an involute type device structure adapted for use on square rather than round die.
It is a further object of the present invention to provide an improved involute type device structure which has superior thermal dissipation characteristics.
It is an additional object of the present invention to provide an involute device geometry which makes maximum use of the available area of a square die by providing a predetermined number of involute arms or branches which intersect the substantially rectangular perimeter of the device in predetermined locations.
It is a further object of the present invention to improve heat extraction from the top surface of the device which has thereon the involute device structure.